Adam PIECZKA 1*, Arkadiusz BUNIAK 2, Jarosław MAJKA 3, Hans HARRYSON 3 - PDF

Journal of Geosciences, 56 (2), DOI:.39/jgeosci.5 Original paper Si-deficient foitite with [4] Al and [4] B from the Ługi- borehole, southwestern Poland Adam PIECZKA *, Arkadiusz BUNIAK 2, Jarosław

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Journal of Geosciences, 56 (2), DOI:.39/jgeosci.5 Original paper Si-deficient foitite with [4] Al and [4] B from the Ługi- borehole, southwestern Poland Adam PIECZKA *, Arkadiusz BUNIAK 2, Jarosław MAJKA 3, Hans HARRSON 3 Department of Mineralogy, Petrography and Geochemistry, AGH University of Science and echnology, Mickiewicza 3, 3-59 Kraków, Poland; 2 Polish Oil and Gas Company (PGNiG S.A.), PGNiG Zielona Góra Branch, Westerplatte 5, Zielona Góra, Poland 3 Department of Earth Sciences, Uppsala University, Villavägen 6, SE Uppsala, Sweden * Corresponding author A tourmaline depleted in Si has been found in the cement of Upper Rotliegend aeolian sandstone overlaying Lower Rotliegend volcanic rock in the Ługi- prospecting borehole near Jarocin in the Fore-Sudetic Monocline, south-western Poland. ourmaline, representing Mg-bearing foitite, crystallized around quartz grains in the form of radial aggregates of tiny crystals, reaching only 2 μm in diameter. Due to the very small size of the tourmaline crystals and the presence of significant contents of transitional metals in the crystal lattice, only direct determination of B 2 in nm-sized spots enables evaluation of [4] Al and [4] B. Compositions of the tourmaline in successive analytical spots show that Si deficiency is supplemented both by [4] Al (. to 8 Al apfu) as well as by [4] B (. to.83 B apfu) at varying proportions. he origin of the mineral was related to diagenesis of evaporate sediments inducing reactions of quartz and clay minerals as primary components in the tourmaline-bearing sandstone with Ca saturated pore brines rich in Cl and (B ) 3. Keywords: Si-deficient tourmaline, tetrahedrally coordinated aluminum, tetrahedrally coordinated boron, Ługi borehole, Poland Received: 25 September 2; accepted: 22 November 2; handling editor: M. Novák he online version of this article (doi:.39/jgeosci.5) contains supplementary electronic material.. Introduction ourmaline represents a supergroup of minerals with highly diversified compositions, whose chemical formulae can be written as X 3 Z 6 [ 6 O 8 ][B ] 3 V 3 W (Hawthorne and Henry 999; Henry et al. 2), where X denotes Na +, K +, Ca 2+, [vacancy]; Li +, Mg 2+, Fe 2+, Mn 2+, Zn 2+, Al 3+, Cr 3+, V 3+, Fe 3+, i 4+ ; Z Mg 2+, Al 3+, Cr 3+, V 3+, Fe 3+, i 4+ ; Si 4+, Al 3+, B 3+ ; B B 3+, ; V OH, O 2 and W OH, F, O 2. Recently, the possibility of Al and B substitution for Si in the tetrahedral site has been one of the most interesting structural problems studied. For many years, it has been accepted that Al can supplement a small deficiency of Si (e.g. Povondra 98). Structure refinements of some crystals representing various members of the tourmaline group corroborate this substitution (Grice and Ercit 993; MacDonald and Hawthorne 995; Bloodaxe et al. 999; Prowatke et al. 23; Ertl et al. 23; Cempírek et al. 26). Boron is one of the main components of tourmaline; however, difficulties in chemical determination of this element result in the common assumption that its content in the tourmaline formula unit is equal to the 3. atoms required to fill the B site completely. Nevertheless, an excess of boron over 3. atoms per formula unit (apfu) has been observed in compositions of some tourmaline crystals for which B 2 was determined directly (e.g. Barton 969; Dyar et al. 994, 998). Such elevated B content indicates that the B site is completely filled by the element, and the surplus has to enter another cationic lattice position. A partial replacement of Si by B was proposed by Barton (969) and, later, by Serdyuchenko (98). Only structural investigations of olenite from Stoffhütte, Austria (Ertl et al. 997; Hughes et al. 2) with a significant excess of boron (close to. apfu) and the lowest measured O mean bond length (.69.6 Å), unequivocally proved that the Si deficiency in tourmaline can be supplemented not only by Al, but also by B. Wodara and Schreyer (997, 998, 2), Schreyer et al. (2) and Marler et al. (22) synthesized an X- site vacant, Al-bearing tourmaline and olenite with significant contents of [4] B reaching c..4 and apfu, respectively. Since then, the presence of [4] B has been noted in many natural crystals of tourmaline rich in Al. he crystals commonly represent olenite (Ertl et al. 997; Hughes et al. 2, 24; Schreyer et al. 22), elbaite (agg et al. 999; Hughes et al. 2), members of the liddicoatite elbaite series (Ertl et al. 26), rossmanite (Ertl et al. 25), an Al-rich tourmaline (Ertl et al. 28) and, only exceptionally, other tourmaline varieties such as schorl (Ertl and Hughes 22) or dravite (Marschall et al. 24). In this paper, we describe extremely fine-crystalline, Sidepleted, Fe-bearing tourmaline found in Permian evaporate Adam Pieczka, Arkadiusz Buniak, Jarosław Majka, Hans Harryson sediments in an oil-prospecting borehole in south-western Poland. o date, only Hancock (978) has noted tourmaline in the Rotliegend sandstone of northwestern Germany. 2. Geological setting he European Rotliegend Basin with numerous natural gas deposits extends from the eastern England coast, through Holland and northern Germany to western and central Poland. his area is very attractive for the gas prospection due to geological and thermodynamic conditions favoring the generation and accumulation of large amounts of gas hydrocarbons, genetically related to the Carboniferous host rocks. In Poland, exploration of natural gas in sandstones of the Upper Rotliegend has been underway since the mid-96 s. Recently, in one of the prospecting boreholes, Ługi-, near Śrem and Jarocin, south-western Poland, Si-depleted tourmaline was found in the cement of the aeolian sandstone (Buniak 29). he Ługi- borehole is situated in the northern part of the Fore-Sudetic Monocline, within the border region of the Upper Rotliegend Basin next to the margin of the Wolsztyn High (Fig. ). Eruptive rocks of the Wielkopolska volcanic formation form the foundation of the sedimentary series of Upper Rotliegend (Pokorski 98). In the nearby boreholes of Dolsk-, Wyrzeka- and Książ Wlkp.-3, rhyolite, dacite and andesite have been found (Jackowicz 994). he overlaying Upper Rotliegend sediments are represented by sandstone corresponding to the Noteć (Pokorski 98) and the Siekierki (Karnkowski 987) formations or the sedimentary sequence 8b (Kiersnowski 997). he sandstone, formed mostly in an arid environment, relates to the final sedimentation stage in the East Erg next to the Wolsztyn High. he transgression of the Zechstein Sea stopped the deposition of sand-drift 6 ' 6 3' 7 ' PLAA LAKE 52 3' Upper Rotliegend (a) playa lithofacies marginal erg and fluvial lithofacies alluvial lithofacies Poznañ aeolian lithofacies 52 5' sediment source areas 7 3' 8 ' 52 5' 52 ' 6 W O L S Z N H I G H Œrem E A S E R N E R G UGI- Jarocin 52 ' C E N R A L B A S I N km AEOLIAN LIHOFACIES ALLUVIAL AND FLUVIAL LIHOFACIES PLAA LIHOFACIES SEDIMEN SOURCE AREAS (b) 5 45' 5 3' POGORZELA HIGH S O U H E R N 2 km E R G WSOCKO H.? Ostrów Wlkp. KALISZ H. Fig. a A map of the Polish Upper Rotliegend Basin with localization of the Ługi- borehole. b Overall geometry of the Polish Upper Rotliegend Basin (after Pokorski 988, modified by Kiersnowski 997). 39 Si-deficient foitite from Poland formations, and the Rotliegend sediments are covered by evaporate rocks of the Werra cyclothem. In the Ługi- prospecting borehole, the Upper Rotliegend sediments, with a thickness of only c. 9.3 m ( m depth range), form a higher part of the Lower Permian formations. Below, up to 27. m, occur Lower Rotliegend volcanics. he sedimentary profile is initiated by reddish brown, fine-grained sandstones and conglomerates deposited in an alluvial environment ( m), resting immediately on volcanics. In the hanging wall occurs horizontally and diagonally bedded, fine- to medium-grained sandstone (7.7 m) grey in colour, which is related to an arid environment. he diagonally bedded sandstone represents the sand-drift core facies, whereas the horizontally and low-angle-diagonally bedded sandstone corresponds to the sand-drift base and inter-sanddrift facies. In the apical part of the complex, there is grey, horizontally bedded, fine-grained sandstone (.4 m) formed in a shallow marine basin. he tourmaline-bearing sandstone is a fine-grained and porous variety with a mean grain diameter of.8 3 mm and moderately to very well sorted out grains, corresponding to quartz and sublithic arenites. he grain facies of the sandstone is composed of pieces of quartz and volcanic clasts, whereas tourmaline, calcite, dolomite, ankerite, anhydrite, halite, quartz and occasionally clay minerals form the cement. An unidentified Ca-chloride (antarcticite, CaCl2 6H2O, or sinjarite, CaCl2 2H2O) in the form of strongly elongated acicular microcrysts was observed in the pores. ourmaline, whose content can be estimated at up to.x vol. % of the bulk rock, is a rather common component in the cement of the sandstone. he tourmaline crystallized around quartz grains (also occasionally around carbonates) in radial aggregates of tiny crystals, reaching μm in length and only 2 μm in diameter (Fig. 2). In the aggregates, tourmaline forms perfect trigonal prisms and parallel intergrowths of two or three crystals. a b µm c µm d ur Carb. µm Qtz 5 µm Fig. 2 Secondary electron microscope (SEM) images of the Ługi- tourmaline (a c) and a BSE image of the Ługi sandstone with numerous tourmaline crystals overgrowing quartz grains and carbonate rhombohedra (d). 39 Adam Pieczka, Arkadiusz Buniak, Jarosław Majka, Hans Harryson 3. Methods Samples for electron microprobe determinations were prepared from the tourmaline-bearing sandstone in a form of -inch epoxy discs, polished and coated with carbon. Initial electron microprobe analyses of the tourmaline have been carried out at the Inter-Institute Analytical Complex for Minerals and Synthetic Substances of Warsaw University with a CAMECA SX electron microprobe, operating in the wave-length-dispersive (WDS) mode under the following conditions: accelerating voltage of 5 kv, beam current of 2 na, focused beam with a diameter of ~ μm, peak count-time 2 s and background time s. he contents of all measured components were calculated in relation to the respective K α analytical lines of the following standards: F phlogopite (AP), Na albite (AP), Mg and Si diopside (AP), Al corundum (AP), Cl tugtupite (PE), K orthoclase (PE), Ca wollastonite (PE), i rutile (PE), Mn rhodonite (LIF), Fe hematite (LIF), Zn sphalerite (LIF), Cr Cr 2 (PE) and V V 2 O 5 (PE). he raw analyses were corrected with the PAP procedure (Pouchou and Pichoir 985) applied by the Cameca. he composition of tourmaline crystals investigated has been normalized in two different ways: () in relation to + Z + = 5 apfu, stoichiometric B = 3 apfu and OH + F + Cl = 4 pfu, assuming the presence of only [4] Al supplementing Si deficiency at the site; (2) in relation to Si + B = 9 apfu and OH + F + Cl = 4 pfu, assuming the presence of only [4] B supplementing Si deficiency at the site. In both cases, Fe 3+ was calculated from the charge balance in the tourmaline formula with 3 (O,OH,F,Cl). he depletion of Si in the tourmaline is clearly visible in its energy-dispersive (EDS) spectra, where the intensity of the AlK α analytical line, for tourmaline commonly comparable with the SiK α line, distinctly predominates in spite of moderate Al contents in that tourmaline. In such cases, the Si deficiency in the site must be supplemented by [4] Al, [4] B or by both cations simultaneously. he recognition of a proper substitution at the site is rather easy based on structural studies. he substitution of Al 3+ for Si 4+ is connected with an increase in the observed O mean bond length from c..62 Å, a mean value typical of the Si O bond, to a higher value due to a much longer [4] Al O bond (~ Å). In contrast, the substitution of B 3+ for Si 4+ results in a decrease in the observed O mean bond length below.62 Å due to a much shorter [4] B O bond length close to only.47 Å (Hawthorne 996). Identification of the replacements is less obvious in the case of the simultaneous presence of [4] B and [4] Al, because the opposite effects of these ions on O mean bond length can balance each other, and the observed O value can also be close to.62 Å, apparently reflecting the presence of 6. Si apfu at the site. Unfortunately, the very small dimensions of the tourmaline crystals rule out the X-ray investigation using the single crystal method. Purification of the tourmaline is impossible in heavy liquids and in any acid mixture. Consequently, a structural study with the use of the Rietveld method could not be done. Magic-anglespinning nuclear magnetic resonance (MAS-NMR) spectroscopy, commonly applied for detecting [4] B and [4] Al in tourmaline (agg et al. 999; Marler and Ertl 22; Marler et al. 22; Lussier et al. 29), also cannot be applied due to the very high presence of paramagnetic Fe ions. Lussier et al. (29) did show that a high content of paramagnetic ions results in a dramatic decrease of intensity, and the recorded spectra are illegible. Ertl et al. (28), studying Al-tourmaline with [4] B, corrected a co-variation between the cell volume of [4] B-bearing tourmalines and [4] B contents in their structures, which was previously proposed by Wodara and Schreyer (2) and Marler et al. (22), allowing for estimation of the [4] B content from X-ray powder diffraction data. However, this relationship is valid only for Al-rich tourmalines, especially olenite, where the site is occupied only by Si and B, without significant [4] Al, and contents of transitional metals are low. For Febearing tourmalines, the relationship is more complex and prevents such estimation. hus, none of the techniques mentioned above commonly used in the detection of [4] Al and [4] B in tourmaline can be used to recognize what kind of Si replacement exists in the Ługi material. herefore, we realized that direct determination of B 2 content is indispensable for further discussion and only possible issue in such impasse situation. Due to the extremely low dimensions of the tourmaline crystals, we accepted only a microprobe determination with the use an apparatus that enables analyses in nm-scale spots. Attempts at direct determination of B content in the tourmaline carried out at the above mentioned laboratory with CAMECA SX microprobe (6 kv, 5 na, PC2 crystal, SRM93a standard, K α analytical line) gave satisfactory results only in a few cases ( wt. % B 2 ). hese suggested the real possibility of a [4] B presence in the material studied. However, in many cases, the determined B content was too low to be accepted, even for tourmaline with stoichiometric B = 3. apfu. Such negative results were probably due to the extremely small size of tourmaline crystals with transversal diameters of only ~ 2 μm; i.e. comparable with a size of analytical spot even with the use of completely focused beam. he porosity of the host rock could have played a role as well. As a consequence, the final direct determinations of B 2 contents were performed at the Centre for Experimental Mineralogy, Petrology and Geochemistry of the Department of Earth Sciences at Uppsala University, Sweden, using a JEOL JXA 853F Field Emission Hyperprobe. he apparatus was operating under the following conditions: accelerating voltage of 5 kv, beam current of 392 Si-deficient foitite from Poland na, focused beam with a diameter of.x μm, counting time of s on peak and 5 s on both (+) and ( ) backgrounds. he contents of all measured components were standardized in relation to respective K α analytical lines of the following standards: B pure B (LDE2), F LiF (AP), Na albite (AP), Si, Ca wollastonite (AP), Mg MgO (AP), Al Al 2 (AP), K orthoclase (PEJ), i pyrophanite (PEJ), Mn pyrophanite (LIFH), Cl vanadinite (PEH), Fe Fe 2 (LIFH), Zn ZnS (LIFH). he data were reduced using the PAP procedure. he obtained tourmaline compositions have been normalized to + Z + B + = 8 apfu with H 2 O and Fe 3+ /Fe total ratio calculated stoichiometrically, assuming (OH,F,Cl) = 4 ions pfu and an electroneutral formula with 3 (O,OH,F,Cl). 4. Results and discussion he complete set of the spot EMP analyses of the Ługi tourmaline performed at Warsaw University is presented in the Electronic supplementary data, statistics for which is given in ab.. he tourmaline shows a low SiO 2 content, only 32.8 wt. % as a mean value, ab. Statistical distribution of of Si-deficient foitite compositions from the Ługi borehole Si-deficient foitite (n = 25) normal foitite (n = 3) IV Al Si IV B Si IV Al Si IV B Si range mean range mean range mean range mean wt. % wt. % SiO B Al Fe MgO CaO FeO Na 2 O H Cl O=Cl otal apfu apfu Na Ca ΣX Mg Fe Al Fe Σ Z Al B Si B Al Σ O OH Cl B 2 contents calculated from stoichiometry Contents of K, i, Mn, V, Cr, Zn and F are below the respective detection limits; n number of analyses. 393 Adam Pieczka, Arkadiusz Buniak, Jarosław Majka, Hans Harryson a Ca.8 Calcic.8 X Site Vacant Alkali.8 X-site vacancy Na+K b Al c Al+ Fe Foitite Magnesiofoitite Foitite Magnesiofoitite Fe tot. Mg.8 2+ Fe Mg Fig. 3 Compositional relationships in the Ługi tourmaline: a the X site occupants; b c the site occupants. Symbols: black diamonds compositions with B 2 content calculated from stoichiometry; white diamonds compositions with direct EMP determination of B 2 contents. and the Al 2 content, wt. %, rather typical of many tourmaline crystals representing schorl, dravite, buergerite, foitite or magnesiofoitite varieties. All spot analyses of the tourmaline indicate a high deficiency in alkali cations, resulting in predominance of vacancy at the X site and, on the other hand, predominance of Fe over Mg and Al at the site (Fig. 3). his suggests unequivocally that the tourmaline has a composition transitional between foitite and magnesiofoitite endmembers and can be classified as Mg-bearing foitite, using a new nomenclature of the tourmaline supergroup (Henry et al. 2). able presents statistics of two excessive cases of tourmaline compositions with the B 2 contents calculated from stoichiometry assuming: () Si-deficiency supplemented solely by [4] Al, (2) Si-deficiency compensated by [4] B. If the presence of only [4] Al substituted for Si is assumed, totals of all EMP analyses of Si-deficient tourmaline range from c to 98.8 wt. % with a mean value of 95. wt. %. On the other hand, analyses in areas 394 Si-deficient foitite from Poland ab. 2 Representative compositions of Si-deficient foitite from the Ługi borehole, with direct EMP determinations of B 2 contents wt. % SiO B Al Fe MgO CaO FeO Na 2 O H F Cl O=F O=Cl apfu X Na X Ca X vac ΣX Mg Fe Al Fe Σ Z Al B Si B Al Σ O OH F Cl corresponding to tourmaline with a low Si deficiency ( normal foitite) sum up to 96.9 to 99.6 wt. %, with a mean of 98.2 wt. %. he same analyses recalculated with an excess of B supplementing the Si deficiency reach totals ranging from 96.4 to.3 wt. % (mean 97.7 wt. %), and 96.9 to.8 wt. % (mean 98.9 wt. %), respectively. he low analytical totals for both types of substitution for Si at the site can be easily explained by the shortcomings of microprobe analyses due to the very small size of the tourmaline crystals, infiltration of epoxy into voids within the cement of the sandstone and analysis of the target material diluted by epoxy. However, the mean totals for Si-deficient and normal foitite are closer to each other in case when [4] B is assumed. his suggests that B could indeed substitute for Si in the tourmaline. Electron microprobe analyses with direct determination of B 2 (ab. 2) corroborate its elevated contents of almost.3 to above 3.6 wt. % in many tourmaline crystals. Chemical formulae calculated for successive spots, based on 3 (O,OH,F,Cl), show that Si deficiency in the Ługi tourmaline is supplemented by [4] Al with [4] B at varying proportions (. 8 Al and..83 B apfu).
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